The island of Samal in the southern Philippines is home to one of 40 sites around the country where giant clams (Tridacna spp.) are nurtured as part of a conservation program.
For the local community, giant clams had long been a source of food, so there was initially some resistance to the program when it started in 2001.
Today, the clam sanctuary has grown into an ecotourism venture that generates revenue for the community and employs local seniors, particularly women.
However, the mollusks are threatened by rising ocean temperatures, declining salinity and other human-driven factors, leaving their fate — and that of the community that has come to depend on them — in the balance.
SAMAL ISLAND, Philippines — Sixty-five-year-old Ruby Gutierrez wakes up at exactly six in the morning, and after preparing breakfast for four grandchildren (she has 13) heads off to the Adecor giant clam nursery, a community-based ecotourism effort on the island of Samal, in the southern Philippine region of Davao, that offers taklobo(giant clam) tours.
Local seniors, especially women, are often excluded from community-based ecotourism efforts as often, they fail to adapt the necessary skillsets required of tour guides, says tourism development expert Carl Milos Bulilan. It’s a different case in Samal’s giant clam sanctuary, however, where five of the 10 guides are aged 60 and above, and three are women. Unlike the men who tow floaters with guests, the ladies oversee orientation and logistics.
When the program started at the turn of the century, Ruby was ushered away from the fields, where she planted root crops, to the teal waters of Davao Gulf to look after giant clams. During the lean months of June to September, the sanctuary gets an average of 60 tourists per day, but the figure peaks at 200 guests during the prime summer months of March to May.
There are 12 identified giant clams species in the Coral Triangle, a swath of the western Pacific rich in marine life, and eight can be found in the Philippines. Four species can be spotted in the Samal sanctuary during an hour’s worth of snorkeling: Tridacna gigas (giant clam), T. derasa (smooth giant clam), T. squamosa (fluted giant clam), and T. maxima (elongated clam).
Managed by the Adecor United Fisherfolk Organization (AdUFOr), the sanctuary is a joint project with the local government and Davao del Norte State College. The nursery is considered a model community project as it balances livelihood needs alongside conservation, says Cecilia Conaco, deputy director of the Bolinao Marine Laboratory (BML), an initiative created by the Marine Science Institute (MSI) of the University of the Philippines, which pioneered the populating and restocking of giant clams in the Philippines in the 1970s.
Initially spawned as an alternative livelihood for fisherfolks, the program took a sharp turn toward conservation after the Fisheries Code of 1998 was enacted. The code banned the collecting, theft, eating, sale and export of giant clams. But mariculture efforts dwindled until various organizations floated the idea of generating revenue through ecotourism.
The first giant clams nurtured in Davao were shipped from BML’s hatchery in Bolinao, Pangasinan province. The clams, once fit for transport in juvenile stages, were flown to 40 areas around the country two decades after the program’s inception.
Thousands of juvenile giant clams arrived on Samal Island in 2001, welcomed by a community that has a long history of serving them for lunch. Initially, the community didn’t sit well with the idea of cultivating the clams. Everything harvested from the waters, says Wilma Diso, 66, another senior tour guide, was meant for the table, especially on days when fish catches were scarce.
After numerous discussions, the ecotourism project yielded positive results. In Samal alone, the community turned its back from eating to protecting the clams. After being tapped to work at the sanctuary, Wilma’s perspective of the clams widened.
“We used to eat the clams, so there was a lot of resistance when this sanctuary started. Even I need to be convinced,” Wilma says. “But this is our work now, the clams feed us. Now I get goose bumps just remembering those days.”
Nursing a dying marine ecosystem
Giant clams are the largest living bivalve mollusks and play a necessary role in creating an abundant marine ecosystem. Numerous studies show that restocking giant clams alongside coral transplantation efforts helps increase the abundance and richness of fish species and improve the condition of degraded patch reefs.
“They attract corals to go into their shells and they become integral proponents to the reef site,” says Conaco.
Prior to the six-year conservation program initiated by MSI, only two T. gigasspecimens were spotted in Philippine waters. Poaching, predation, fouling of cages, typhoons and water temperature anomalies were prime causes of giant clam mortalities.
Due to their low population within the Coral Triangle, giant clams are listed in Appendix II of the Convention on the International Trade of Endangered Species of Wild Flora and Fauna (CITES) and are classified as vulnerable on the International Union for Conservation of Nature’s (IUCN) Red List.
These clams can grow as large as 1.5 meters (5 feet) and serve an important role in their ecosystem of keeping algal growth in check, filtering the seawater, recycling nutrients, and acting as a habitat for small organisms, as their tough shells help form coral reef structures.
Giant clams’ contribution to the marine ecosystem has made them a hot commodity for local governments and private resorts. “We started getting requests from resorts and local government units who are interested in putting clams in their areas to attract tourism … and maybe to improve their marine protected areas,” Conaco says.
On Samal Island, locals directly attribute burgeoning fish catches to the presence of the clams. But Conaco says the MSI has yet to formalize this observation. Some local governments have also imposed fishing restrictions in response to dwindling catches, which may account for a rebound in fish stocks.
“We’re trying to make scientific connections. We want to know: Is it really the clams or is it because there are corals that are attracted to the clams? Or is it simply because there’s more structure?” Conaco says. “We’re trying to see what attracts the fish to areas where the clams are.”
Adapting, but until when?
There’s still a lot to learn about the clams, says Conaco, noting that the bivalves adapt well to new environments. Giant clams’ colors tend to vary depending on areas. In Bolinao, in the northern Philippines, they’re brown with green edges; in the prime diving site of Anilao, Batangas, further south, they sport blue spots. Elsewhere, they’re perfectly camouflaged in the reefs and can only be seen with a trained eye.
But despite the clams’ high adaptability, rising sea temperatures, ocean acidification, and reduced salinity due to climate change and other man-made causes are having dire impacts on their development and survival.
“High temperature can be detrimental for the clams because like corals, they bleach,” Conaco says. “They lose algae in the tissues and the shells whiten. We’re still trying to see if they can recover if conditions normalize.”
A study by Conaco and her team in 2018 showed that developmental abnormalities occur when the clams’ embryonic and larval stages are exposed to high sea temperatures. While clams in the larval stages developed faster, they also exhibited lower survival rates. The study exposed the giant clams to seawater varying in temperature from 28 to 33 degrees Celsius (82 to 91 degrees Fahrenheit).
“Although embryonic and larval development was slower at 28C compared with 30C and 33C, more larvae developed normally and survival was greater at the lower temperature compared with the higher temperatures,” the study says.
Even if giant clams can adapt to rising sea temperatures, other factors contribute to low survival rates in their larval stage. A 2019 study found that reduced salinity affects both fertilization success and the survival of developing larvae. No larvae survived beyond two days of exposure to low-salinity conditions.
Increased precipitation due to changing weather patterns, sewage disposal, agricultural runoff and other factors leads to a reduction in salinity levels of shallow-water reefs, which in turn reduces the fertilization success of giant clams.
Should precipitation intensify, giant clams are less likely to survive in reefs, the study states.
“There’s so many aspects when it comes to climate change impacts, but to a certain extent … in the next couple of years we’ll probably know more at least for certain stages of the clams,” Conaco says. She adds that the MSI has yet to conduct similar studies on mature clams.
Back on Samal Island, the day’s watch ends at 5 p.m. for Ruby, Wilma and the other managers of the giant clam sanctuary. They look after 2,196 clams now, significantly less than the estimated 3,700 that were transplanted here when the sanctuary opened. Some died of sickness, says Wilma, the others simply disappeared, probably taken by poachers who tiptoed in during the middle of the night.
Gomez, E. D., & Mingoa-Licuanan, S. S. (2006). Achievements and lessons learned in restocking giant clams in the Philippines. Fisheries Research, 80(1), 46-52. doi:10.1016/j.fishres.2006.03.017
Cabaitan, P. C., Gomez, E. D., & Aliño, P. M. (2008). Effects of coral transplantation and giant clam restocking on the structure of fish communities on degraded patch reefs. Journal of Experimental Marine Biology and Ecology, 357(1), 85-98. doi:10.1016/j.jembe.2008.01.001
Enricuso, O. B., Conaco, C., Sayco, S. G., Neo, M. L., & Cabaitan, P. C. (2018). Elevated seawater temperatures affect embryonic and larval development in the giant clam Tridacna gigas (Cardiidae: Tridacninae). Journal of Molluscan Studies, 85(1), 66-72. doi:10.1093/mollus/eyy051
Sayco, S. G., Conaco, C., Neo, M. L., & Cabaitan, P. C. (2019). Reduced salinities negatively impact fertilization success and early larval development of the giant clam Tridacna gigas (Cardiidae: Tridacninae). Journal of Experimental Marine Biology and Ecology, 516, 35-43. doi:10.1016/j.jembe.2019.04.004
Cover photo: T.gigas were virtually extinct in the Philippines in the 1970s but decades of conservation bore significant increase in population. While still classified as vulnerable, t.gigas are often transplanted in marine protected areas and dive sites by local governments and private resorts. Image courtesy of Bolinao Marine Laboratory.